package main

import (
	"fmt"
	"log"
	"os"
	"regexp"
	"strings"

	git "github.com/go-git/go-git/v5"
	"github.com/go-git/go-git/v5/plumbing"
	"github.com/go-git/go-git/v5/plumbing/object"
	"github.com/go-git/go-git/v5/plumbing/storer"
	gha "github.com/sethvargo/go-githubactions"
)

var rxHeader = regexp.MustCompile(`^(\[\#[0-9]+\]\s|Release|Revert|Reapply)`)

func main() {
	// Prepare regexp templates.
	rxSignOff := regexp.MustCompile(`^Signed-off-by:`)

	// Open current git dir.
	r, err := git.PlainOpen("./")
	if err != nil {
		log.Fatalf("Failed to open repository: %v", err)
	}

	// Retrieve the commit history.
	head, err := r.Head()
	if err != nil {
		log.Fatalf("Failed to retrieve HEAD reference: %v", err)
	}

	// Create iterator over commits.
	commits, err := r.Log(&git.LogOptions{From: head.Hash()})
	if err != nil {
		log.Fatalf("Failed to retrieve commit history: %v", err)
	}

	// Limit number of iterations.
	from := gha.GetInput("from")
	log.Printf("Resolving revision '%s'", from)

	var lca *object.Commit
	if from != "" {
		lca = getMergeBase(r, head, from)
	}

	log.Printf("Resolved: %s", lca.Hash)

	// Processing result.
	var fail bool

	_ = commits.ForEach(func(c *object.Commit) error {
		// Stop iterator when limit is reached.
		if lca != nil && c.Hash == lca.Hash {
			return storer.ErrStop
		}

		// Parse commit data.
		id := c.ID().String()[:7]
		lines := strings.Split(strings.Trim(c.Message, "\n"), "\n")

		// Do not process empty commit.
		if len(lines) == 0 {
			fail = true
			fmt.Printf("Error: empty commit [%s]\n", id)
			return nil
		}

		// Check commit header.
		header := lines[0]
		if !rxHeader.MatchString(header) {
			fail = true
			fmt.Printf("Error: invalid header '%s' [%s]\n", header, id)
			return nil
		}

		// Check empty line after header.
		if 1 < len(lines) && lines[1] != "" {
			fail = true
			fmt.Printf("Error: second line must be empty '%s' [%s]\n", lines[1], id)
			return nil
		}

		// Check commit sign-off.
		if !rxSignOff.MatchString(lines[len(lines)-1]) {
			fail = true
			fmt.Printf("Error: missing sign-off '%s' [%s]\n", header, id)
			return nil
		}

		fmt.Printf("OK: '%s' [%s]\n", header, id)
		return nil
	})

	// Return non-zero code if DCO check failed.
	if fail {
		os.Exit(1)
	}
}

func getMergeBase(r *git.Repository, head *plumbing.Reference, from string) *object.Commit {
	h, err := r.ResolveRevision(plumbing.Revision(from))
	if err != nil {
		h, err = r.ResolveRevision(plumbing.Revision(plumbing.NewBranchReferenceName(from)))
		if err != nil {
			log.Fatalf("Failed to resolve a reference '%s': %v", from, err)
		}
	}

	to, err := r.CommitObject(*h)
	if err != nil {
		log.Fatalf("Failed to get commit object: %v", err)
	}

	other, err := r.CommitObject(head.Hash())
	if err != nil {
		log.Fatalf("Failed to get HEAD commit object: %v", err)
	}

	cc, err := to.MergeBase(other)
	if err != nil {
		log.Fatalf("Failed to determine merge-base: %v", err)
	}

	return cc[0]
}